This invention relates broadly to workpiece positioning systems, and more specifically to high-precision, X-Y position measurement apparatus employed with feedback-controlled positioning apparatus in step-and-repeat mask aligners for fabricating semiconductor integrated circuits.
Integrated circuit fabrication involves successively forming a series of microcircuit patterns upon a semiconductor substrate. Machines known as mask aligners are used to transfer each microcircuit pattern from a mask or a reticle to a photoresist-coated surface of the substrate. Subsequent processing of the substrate involving etching, deposition, ion implantation, or other similar techniques forms each transferred microcircuit pattern on the substrate. Each of the series of microcircuit patterns formed on the substrate must be accurately aligned with respect to all other previously-formed microcircuit patterns to insure a functional integrated circuit.
One type of mask aligner known as a step-and-repeat projection mask aligner, or stepper, is commonly used in the semiconductor industry today. Such a stepper optically projects an image of a mask or a reticle, incorporating a microcircuit pattern for a single die or a small number of dice, onto a portion of a photoresist-coated surface of the substrate. The microcircuit pattern is photolithographically printed on that surface portion of the substrate by exposing that surface portion to ultraviolet light passing through the mask or the reticle. The substrate is thereupon successively stepped to different positions so that the same microcircuit pattern may be repeatedly photolithographically printed on adjacent surface portions of the substrate in the same manner across the entire photoresist-coated surface of the substrate. Images of the mask or the reticle may be projected by employing a 1:1 projection lens or by employing a reduction, such as a 5:1 or a 10:1, projection lens.
There are certain advantages, and disadvantages as well, to the utilization of step-and-repeat mask aligners, or steppers, as opposed to other types of mask aligners, such as contact-printing and proximity mask aligners. One advantage is that steppers can use masks or reticles for exposing only a fraction of the total photoresist-coated surface of the substrate at one time, whereas contact-printing and proximity mask aligners must use masks for exposing the entire photoresist-coated surface of the substrate at once. Thus, the masks or reticles used by steppers are easier to produce than the masks used by contact-printing and proximity mask aligners. Another advantage of reduction type steppers is that they may be less sensitive to particulate contamination of the mask or reticle than 1:1 steppers or other mask aligners. However, one disadvantage of all steppers is the time required for the multiple exposures necessary to completely expose the photoresist-coated surface of the substrate.
In order to compete with the production throughput of contact-printing and proximity mask aligners, steppers require rapid and accurate repositioning of the substrate between exposures. This is typically accomplished by employing position measurement apparatus with feedback-controlled servomotor positioning apparatus to drive a moveable stage, upon which the substrate is supported, from one exposure position to another along orthogonal X and Y axes. Accurate position measurement apparatus must be utilized to achieve the sub-micrometer positioning accuracy necessary for microcircuit pattern alignment and to facilitate precision stepping between exposure positions. Such precision stepping can increase production throughput by eliminating the need for time consuming substrate-to-mask or reticle alignment operations between exposures.
In the past the position measurement apparatus has often included a laser interferometer utilizing the wavelength of a laser beam to measure or determine stage and, hence, substrate position. Inaccuracies are inherent in this technique, however, since the wavelength of a laser beam is a function of temperature and barometric pressure. To achieve sufficiently accurate position feedback, the laser beam must be housed in an environmentally-controlled chamber, which adds substantially to the complexity and cost of a stepper so equipped.
Thus, an improved positioning system is needed for accurately and rapidly positioning a semiconductor substrate, or more generally any workpiece, at a sequence of predetermined positions. In addition, an improved workpiece positioning system is needed that is capable of retaining sufficient positioning accuracy throughout a range of environmental conditions typically experienced by workpiece positioning systems.
Workpiece positioning systems of a type generally similar to the present invention are described in U.S. Pat. No. 4,425,537 entitled X-Y ADDRESSABLE WORKPIECE POSITIONER AND MASK ALIGNER USING SAME and filed on Oct. 20, 1980 by Edward H. Phillips and Karl-Heinz Johannsmeier (a continuation of U.S. patent application Ser. No. 918,713 filed on June 26, 1978, and now abandoned) and in U.S. Pat. No. 4,442,388 entitled X-Y ADDRESSABLE WORKPIECE POSITIONER HAVING AN IMPROVED ADDRESS INDICIA SENSOR and filed on July 27, 1981, by Edward H. Phillips (a continuation of U.S. patent application Ser. No. 136,816 filed on Apr. 2, 1980, which is in turn a continuation of U.S. patent application Ser. No. 925,454 filed on July 17, 1978). U.S. Pat. Nos. 4,425,537 and 4,442,388 are assigned to the same assignee as the present application and are herein incorporated by reference. The workpiece positioning systems described in these patent applications constitute the closest prior art known to applicants and comprise the following elements: a workpiece stage moveable along coordinate X and Y axes; an X-Y array of coordinate indicia attached to the workpiece stage; a light source for illuminating the coordinate indicia; a sensor stage moveable along the coordinate X and Y axes; an opaque masking plate mounted on the sensor stage and provided with transparent windows; a lens system for projecting a focused image of the coordinate indicia onto the masking plate; an array of photodiodes mounted on the sensor stage and responsive to light reflected from the coordinate indicia and transmitted through the windows of the masking plate; a first pair of servomotors and first control apparatus for controlling the position of the workpiece stage; and a second pair of servomotors and second control apparatus for controlling the position of the sensor stage.
To position a workpiece at a desired position, the first pair of servomotors and the first control apparatus move the workpiece stage and, hence, the workpiece to a position at which the positional error from the desired position is no greater than the indicium spacing. The first and second control apparatus then couple, or lock, the workpiece stage to the sensor stage for movement therewith. This permits the sensor stage to be moved to an interpolated position so as to concomitantly move the workpiece stage to the desired position. Linear variable differential transformer displacement transducers are utilized for providing sensor stage position feedback.
While the above-described workpiece positioning systems are fully functional, they are limited in workpiece positioning accuracy by the accuracy of the linear, variable differential transformer displacement transducers. Additionally, the fabrication of these workpiece positioning systems is quite complex due to the alignment requirements of the sensor stage relative to the workpiece stage and due to the motion and control requirements of the sensor stage. Thus, a more accurate and less complex workpiece positioning system is needed.